Giant functioning parathyroid cyst presenting as a retrosternal goitre.

Parathyroid cysts are rare and even more rarely cause a neck mass resembling a goitre. Such large parathyroid cysts may involve the mediastinum, growing to a sufficient size to produce symptoms related to obstruction, and if functioning, primary hyperparathyroidism. Parathyroid cysts should be considered in the list of differential diagnoses of anterior neck masses to allow for appropriate preoperative investigation to avoid unnecessary confusion at the time of operation. We report a case where a functioning parathyroid cyst presented as a retrosternal goitre to emphasize the potential pitfalls associated with their diagnosis and management.